Kallistan, from Ultima Underworld
When the Avatar freed him from his cell in Ultima Underworld I on the seventh level of the Abyss, Kallistan was mistrustful, until the Avatar proved to know Goldthirst by giving his password "Deco Morono".
Kallistan gave the Avatar the Crystal Splinter, explaining that he'd found it during his forced mining labor, and that it could open a secret crypt to create shortcuts through the seventh level. He warned the Avatar that the tomb had been defiled by evil, and that the hero shouldn't disturb the peace of the dead buried there.
Later, Garamon rescued him when the volcano erupted.
Apparently, if you kill Tyball before entering the jail, Kallistan disappears, and the hidden areas become inaccessible.